Known is an inverted pendulum type vehicle or an omni-directional vehicle incorporated with a frictional drive device which comprises a pair of drive assemblies individually actuated by electric motors and a main wheel held between the drive assemblies and frictionally driven by the drive assemblies. See WO2008/132779A1 (US20100096905A1) for instance. Each drive assembly comprises a drive disk coaxially opposing the drive disk of the other drive assembly and a plurality of drive rollers obliquely arranged along the circumference of the drive disk at a regular interval so as to be individually rotatable. The main wheel comprises a ring-shaped annular member rotatably supported by a frame around a central axial line thereof and a plurality of driven rollers arranged along the circumference of the annular member so as to be rotatable around the respective tangential lines. As the drive disks are turned by the electric motors, the driven rollers are frictionally driven by the drive rollers. When the drive rollers are turned around the tangential directions of the main wheel, the vehicle is driven in a lateral direction. When the main wheel is turned around the central axial line thereof, the vehicle is driven in a fore and aft direction. The direction of motion of the vehicle can be selected as desired by suitably adjusting the difference between the rotational speeds of the two drive disks.
In such a frictional drive device, the driven roller or driven rollers engaging the road surface or the object to be actuated are engaged by at least one of the right drive rollers and at least one of the left drive rollers so that the traction force or drive force can be obtained at all times. In this regard, it is highly important that the slippage between the drive rollers and driven rollers to be minimized in achieving a high efficiency of the frictional drive device and increasing the traction or drive force which the frictional drive device is capable of delivering.